Hi OKG,
I assume that you are sending from a network in a large organisation such as a university or a large corporate network ?
It appears to be an anti spam feature used by organisations which prevents their mail servers from spamming people.
See this article :
I'm sending email from another machine on the network and I'm getting "550 Relaying denied" errors.
This is due to the new version of sendmail now being shipped. This contains spam filtering and will block use of the mail server as a mail relay to stop people attempting to send spam via your server. The mail server will only accept email that is destined for somewhere in the outside Internet) from specified machines.
When mail is sent to the server, it checks the identity of the machine sending the mail either by name or IP address. The file /etc/relay-domains contains either partial IP addresses (for instance, 192.168.2 to match all IP addresses from 192.168.2.1 to 192.168.2.254) or a domainname. Early NetManagers (and NCManagers from Xemplar) checked by name; modern NetManagers check by IP address. Checking by IP address is much simpler as machines do not need to be given names on the server.
Firstly, check the contents of /etc/relay-domains. If it contains your local domain name (e.g. school), it should be altered to contain a partial IP address as follows:
Determine your local IP address range
Type: rm /etc/relay-domains
Type: joe /etc/relay-domains
Enter the parts of the IP address that are common to all machines on your network (e.g. 192.168.2 if all your machines are 192.168.2.x or 10.3 if all your machines are 10.3.x.y)
Type Ctrl-K followed by X
Reboot (or type: kill -HUP `sed 1q /var/run/sendmail.pid`)
If mail is still not being sent when you are checking by IP address, then the machine you are sending mail from must be external to your network (e.g. on your admin network on a different range of IP addresses). In such a case, edit the /etc/relay-domains file (as above) to add the extra IP address range.
I hope that this helps